1. Field of the Invention
This invention relates to a thermal head comprising a protection layer having a printing surface which is brought into contact with a heat sensitive record medium, a heat generating section which includes heat generating resistors and electrodes connected to the heat generating resistors and generates heat transferred to said heat sensitive record medium through said protection layer, and a driving circuit connected to said electrodes for supplying a heating electric power to the electrodes.
The present invention also relates to a method of manufacturing the thermal head of the kind mentioned above.
2. Related Art Statement
The thermal head of the kind described in the preamble has been used in simple and low cost printers using heat sensitive papers and heat transfer papers which do not require a supply of inks. In printers using such a thermal head, a high image quality and high printing speed have been required. For instance, in a heat transfer type color printer or an index printer installed in an automatic mini-laboratory, a thermal head having a very high resolution such as 600 dpi to 1200 dpi has been required.
However. in such a thermal head, an excellent heating up and cooling down property is required in order to raise a temperature of the heat generating section within a very short time and to dissipate heat at a high rate. Such a high speed heating up and cooling down property is particularly required for avoiding undesired blur in an printed image. In order to attain a prompt heating up, it is required that a generated heat does not escape from the heat generating section, and in order to effect the rapid heat dissipation, a generated heat has to be dissipated as soon as possible. For attaining a desired heating up and cooling down property, these two contradictory problems have to be solved simultaneously.
Various requirements for the thermal head may be summarized as follows.
1) small size, light weight, simple structure
2) low price
3) large image size covering A3 size
4) low power consumption
5) high printing speed
6) high density and high resolution
7) uniform image quality without irregular color
In order to attain one or more of the above mentioned characteristics, there have been proposed various thermal heads. For instance. in Japanese Patent Kokai Hei 5-64905, there is proposed a known thermal head shown in FIG, 1. In this known thermal head, a printing surface is formed to be flat, and therefore a space is hardly formed between the printing surface and a heat sensitive record medium and a thermal efficiency is improved. As shown in FIG. 1, on a flat surface of a preliminary substrate 30, a pealing-off layer 31, a wear and abrasion resistance layer 32, a protection layer 33, a heat generating resistance layer 34, an electrode layer 35 and a heat storage layer 36 made of polyimide resin are successively applied. After cementing the assembly to a substrate 38 by means of an adhesive layer 37, the preliminary substrate 30 is removed by means of the pealing-off layer 31. In this manner, a flat printing surface 40 can be obtained.
However, in the known thermal head illustrated in FIG. 1, in which the heat storage layer 36 is made of a resin material such as polyimide resin and the printing surface 40 constituted by a portion of the wear and abrasion resistance layer 32 is formed to be flat, has the following problem.
A record paper is urged against the thermal head with a very strong force by means of a press roller, but since the printing surface is flat, the roller is brought into contact with the thermal head over a larger area and thus the pressing force is decreased. This results in that an influence of the roller deformation and abrasion might occur.
In order to mitigate the above problem, the inventors have proposed a thermal head, in which a printing surface is curved outwardly or is protruded from one surface of a protection layer and a driving IC is provided on the other surface of the protection layer. However. a thickness of assembly besides the protruded printing surface becomes small, and therefore a mechanical strength is decreased and a reliability of the thermal head is reduced.